The present invention relates generally to an electronic ballast for powering a discharge lamp. More particularly, the present invention relates to an electronic ballast with controlled filament preheating using half-wave lamp current detection.
An electronic ballast as known in the art for lighting a hot-cathode type discharge lamp, such as a high-pressure discharge lamp, also referred to as an HID (High-intensity discharge lamp), typically includes a power converting circuit for receiving a DC power input and outputting AC power, and a control circuit for controlling the power converting circuit.
In an example of an electronic ballast of this type as previously known in the art, the control circuit performs a filament heating operation by increasing the output frequency of the power converting circuit to heat each filament of the discharge lamp upon startup of the discharge lamp. This heating operation takes place after a starting operation wherein the output voltage of the power converting circuit is relatively higher, and before the start of steady-state lamp.
The electronic ballast of this example can stabilize lamp discharge after a shift to the steady-state operation and reduce lamp fade-out as compared to the case where filament heating not performed.
Referring to FIG. 24(a), a starting period P1 is shown during which a starting operation is performed. When a subsequent filament heating period P2 (during which a filament heating operation is performed) is relatively short as shown, the filament of the discharge lamp is not sufficiently heated before the start of a steady-state period P3. This results in a current being output to the discharge lamp I_lamp (hereinafter referred to as “lamp current”) becoming asymmetrical between polarities. When the filament heating operation shifts to the steady-state operation wherein the filament of the discharge lamp is not sufficiently heated, as described above, discharge becomes unstable after a shift to the steady-state operation, thereby possibly causing lamp fade-out. Accordingly, it is necessary to set the filament heating period P2 to be sufficiently long as shown in FIG. 24(b). The necessary length of the filament heating period P2 (duration of the filament heating operation) varies for each discharge lamp.
However, where multiple discharge lamps may be connected to the ballast, a particular duration of the filament heating operation that accommodates all discharge lamps is likely to be excessive for some lamps. Because the filament heating operation allows the power converting circuit to output larger power than during steady-state operation, to inhibit a negative effect on the life of the discharge lamp, the duration of the filament heating operation needs to be reduced as much as possible while yet properly heating the filaments.